In the first and second of installment we examined the evolution of transistor ignition from being a high performance or fleet oriented option, into being a mainstream ignition to help meet the ever tightening emissions regulations of the early 1970s. Although, General Motors was one of the pioneers of electronic ignition and it had the most advanced ignition designs with the most variations, it was the last of the big three to bring its electronic ignition to the mainstream. Nevertheless, GM had the most extensive experience with electronic ignition, so when it went mainstream it brought the most advanced ignition system of the big three to the market – the High Energy Ignition (HEI) system (also discussed in detail by our own Daniel Stern).
The HEI system was introduced in late 1974 on some GM cars, and was used across the entire GM product line in 1975. This new ignition had a significantly higher output and longer spark duration, assisting GM meet the ever tightening emission standards. The HEI design team had several objectives for the ignition system. These were:
- A maintenance free distributor, coil and electronics
- Extended spark plug life
- Improved combustion of fuel mixtures by increasing spark duration by 50% and spark voltage to 35 kV
- Improved reliability and improved insulation materials
Several of the design innovations used on the Delco-Remy Unitized ignition were carried forward. The success of the Unitized Ignitions integral design meant that from the beginning it was planned the HEI system would also be an integral design. The advantage of the integral design was that it allowed for testing the ignition system prior to installation in the vehicle, improving quality control and production efficiency. Vehicle installation was also simplified, requiring only connecting the spark plug wires, the 12 volt power supply and adjusting the timing.
The HEI distributor was similar to Unitized design, and continued to use the same basic design for its magnetic pickup that Delco-Remy designed in the early 1960s. The HEI distributor increased in diameter, which was required to prevent cross fire due to the higher voltage. New improved insulating materials were used for the distributor cap and rotor to help withstand the extra electrical stress. The larger diameter distributor allowed for the module to be placed inside the distributor under the cap, rather than outside of it as on the Unitized Distributor.
Most of the HEI system was just an evolution of past Delco-Remy designs, but ignition the module was a revolutionary component. Before we examine the HEI module, it’s important to look at the weaknesses of the previous transistorized ignitions. Due to the high stresses from the extreme conditions that an automotive ignition endure, it wasn’t possible to make full use of the transistors capability in previous electronic ignitions. Unlike electro-mechanical parts (such as ignition points), the transistors used in ignitions were not capable of handling any undue stresses caused electrical current spikes. These spikes can occur when starting a car, in the brief period when the engine started but the ignition switch is in the “run” position. Electronic components can immediately fail under these extreme conditions. Therefore, a significant safety margin well below the components capability was engineered in place. This was typically done through a reduction in power through a resistor. As a result, while transistor ignitions were often more capable than standard breaker point ignitions in most circumstances, the margin overall improvement in performance wasn’t always as big as one might assume.
For the engineers to meet the performance goals required a solution that would allow for the electronics to cope with the electrical spikes. That solution was current regulation. The new HEI module regulated the current to a maximum of 5.5 amps. This prevented the transistors from being damaged from excessive current and allowed for the transistors to perform near their maximum capability.
One of the major reasons that an ignition coil has poor high RPM performance is caused by the resistance in the primary circuit. Breaker-point ignitions uses a coil with high primary resistance to limit the current at lower engine speeds, but it comes at the cost of high engine speed performance. The HEI system used a new low resistance coil, with a resistance value of 0.5 ohms (compared to a 2.6 ohm coil for a typical breaker-point ignition). The lower resistance coil also allows for much faster charging of the ignition coil and it can create more voltage with less amperage. To compare, a standard 2.6 ohm coil would require 11 amps to produce 35kV, while a 0.5 ohm coil produces 35kV with only 5.5 amps.
The coil was also redesigned to be more like a traditional transformer than the old canister style coils. The new “e-coil” had the primary winding wound around the inside and the secondary windings on the outside, the opposite of a canister coil. This allowed a low primary resistance with a reasonable size of wire. The entire assembly is potted in epoxy to better protect it against moisture and degrading effects compared to an oil filled coil.
The current limiting ignition module is what works hand in hand with this low resistance coil. The module ensures that at low speeds the current is limited to prevent excessive current stress, while the low resistance coil drastically improves ignition performance at higher engine speeds.
The HEI module also had one further revolutionary feature, automatic dwell time control. Unlike past ignitions that had a fixed dwell time, the HEI module was designed to turn on the coil current when there is just enough time for the coil to reach full charge. By minimizing the dwell time to just what is required, this function eliminates excessive power dissipation, which in turn reduces the module operating temperature and increases its reliability.
The module turns on the coil current to charge the coil when the pickup coil in the distributor is creating a the slower positive portion of the AC wave form. From this portion of the wave form, the module anticipates the next ignition pulse. The module adjusts its turn-on point so there is just enough time for the coil to charge before the primary circuit is opened. So, the coil turn-on occurs just prior to ignition at low speed and advances toward the previous ignition pulse at high speed (as seen in diagram above). The result was the ignition produced a consistent 35kV of output voltage up to 3100 RPM (where it begins to decrease) and 30 kV of output voltage during cranking with battery voltage as low as 6 volts.
The HEI system met all of its design goals and it was the most advanced of all of the ignition systems to come from the Detroit Three in the 1970s. The HEI system allowed for significantly longer spark plug life and wider spark plug gaps, improve starting, reduced emissions through more complete combustion, and reduced maintenance with consistent ignition performance over longer periods. Today, although HEI has long been superseded, it remains very popular to retrofit an HEI module in all sorts of other ignition system, as we saw in tbm3fans’ 1973 Polara. Further, the distributor design has been copied many times over in the aftermarket for countless applications, allowing for simply “one wire” ignition installations on all kinds of engines, including many non-GM engines.
Chrysler was first to offer a mainstream electronic ignition system, but it wasn’t long before this system was further enhanced when it was incorporated into Chrysler’s Lean Burn ignition. Although the ignition was a key component in Chrysler’s Lean Burn System, it also involved several other components. These were a specially designed Carter Thermo-Quad carburetor and numerous sensors for throttle, air temperature, manifold vacuum, and coolant temperature. Controlling this entire system was the Spark Control Computer, which was attached to the side of the air cleaner. Inside the computer were actually two modules, the ignition control module and the program schedule module.
At first glance the ignition system for a Lean-Burn car didn’t appear much different than that the Chrysler EIS. However, the distributor was significantly changed. The vacuum advance was eliminated and the distributor used two pickup coils. Instead of the slower and less accurate vacuum advance canister controlling the ignition advance curve under cruising conditions, this was job was controlled by the timing circuit in the spark computer. Through the readings the computer obtained from the various sensors the ignition timing could be precisely controlled to help burn lean and cleaner fuel mixtures.
Each of the two pick-up coil in the distributor had a specific purpose. One pick-up coil was used for starting the engine and the second was used for when the engine was running. The start pickup uses a fixed timing setting only. Once the engine starts, the “run” coil pickup takes over and it is controlled by the spark control computer which can control the ignition advance almost instantaneously. If the “run” pickup ever failed, the engine could operate solely off the “start” pickup, although the timing would be fixed.
The Spark Control Computer’s program schedule module determines the optimal ignition timing that is required based on all the input from the sensors. It provides the correct amount of ignition timing required for the engine under its current operating conditions. The other half of the Spark Control Computer is the ignition control module. This controls the ignition’s primary circuit at the coil, just as it did in the Chrysler EIS. The program scheduler alters the timing of when the signal is sent to ignition control module to interrupt the ignition coil’s primary circuit. By controlling the timing of the signal for opening the primary circuit, the Spark Control Computer can advance or retard the timing as required, and can do so much more quickly and precisely than a vacuum advance canister. It should be noted that the spark control computer didn’t have absolute control over the timing advance, as the distributor still used a centrifugal mechanical advance.
Chrysler introduced the Lean-burn System for the 1976 model year of the 400-4bbl engine in intermediate and full-size cars. In 1977 the second generation Lean Burn was released and it became available on all V8 engines. It had a few revisions that made the system a bit simpler. Chrysler eliminated the dual coil pickups in the distributor and went back to a single pickup. It also combined the two modules in the Spark Control Computer to be one unit. And finally the Spark Control Computer was give full control over the timing curve, as the mechanical advance was eliminated from the distributor.
The Lean Burn System did not increase the overall ignition energy over the previous EIS. However, it was the first US-made ignition to offer precise computer control of the advance curve. While previously an ignition system could only control its advance via engine speed, and load, the Lean Burn system allowed for input from eight sensors to determine the optimal timing for more specific conditions, which in turn permitted a leaner cleaner fuel mixture to be burned. The end result was reduced emissions, with less rudimentary external emissions devices. However, as it was later discovered, Chrysler’s idea of mounting the Spark Control Computer on the air cleaner proved to be a poor idea for the primitive electronics within and the system did not hold up well over time. Reliability was definitely not the strong suit of the Lean Burn System, but it still should be remembered for pioneering such advanced technology for the era.
Ford’s Solid State Ignition introduced in late 1973 was not cutting edge in anyway, being very similar to Chrysler’s EIS. Ford “refined” the ignition system on an annual basis between 1974 and 1976, but only after three years of Beta testing did the introduce the final product in 1977 – the DuraSpark. Ford released two variations of DuraSpark in 1977, DuraSpark I, used exclusively in California and DuraSpark II used for the remaining cars. While each system was similar, there were a few noteworthy differences.
The much more common DuraSpark II system was very similar to the Solid State Ignition that preceded it, but it had a number of improvements that improved performance. Ford reduced the primary resistance in the resistor wire which increase the primary voltage. This resulted in higher secondary voltage, improved ignition performance and increased overall ignition energy. This in turn helped produce a cleaner burning ignition system. The internal distributor components were unchanged, but Ford increased the diameter of the distributor cap and switched it to male terminals. Like GM’s HEI, the larger diameter was used to help prevent crossfires due to the higher voltage of the ignition system. Ford carried over the same ignition module and coil from the 1976 Solid State System, both of which remained in use for the life of DuraSpark II. Although an improvement over the Solid State ignition, DuraSpark II was not as powerful as the GM HEI system, and did not have advanced features such as an ignition module with current limiting or dynamic dwell. It did however, incorporate some basic timing control, and allow the ignition timing to be retarded when the car was being started, to allow for easier starting.
Due to the much more strict emissions in California, Ford designed a more advanced ignition system, solely for that market – DuraSpark I. This ignition had the same basic design and components of the DuraSpark I, but had several important changes. First, like GM’s HEI system, Ford eliminated the resistance wire allowing a full 12 volts to be applied to the coil which increased the primary and secondary voltage. Ford also added a low resistance coil, 0.70 ohms and a special module incorporated dynamic dwell. With these changes, the DuraSpark I system was more similar to the GM HEI system, with the exception being that it was not an integral design.
DuraSpark I was short lived. After 1977, only 302 powered models in California used this ignition as Ford was able to make the other engines pass emissions using the less costly DuraSpark II system. After 1979, DuraSpark I was dropped. Further, DuraSpark I proved to be not the most reliable ignition in the field. DuraSpark II, on the other hand, remained in use though until 1986. Ford did make several varieties of modules, some for special applications like high altitude, but most used the common and reliable “blue strain” module. DuraSpark II proved to be a reliable ignition, and today many Ford enthusiasts retrofit DuraSpark II to their older vehicles.
With that, we’ve reached the end of the decade and the end of the series. These last ignition systems represented the most advanced non-computer controlled ignitions (save for the Lean-Burn). GM’s HEI, Ford’s DuraSpark and Chrysler’s EIS all remain commonly used today by the old car enthusiasts due to their improvement over a points ignition and simple, reliable designs compared to aftermarket ignition. In the next decade, computer control ignitions moved to the forefront and each of the Big Three’s ignitions were adapted. The GM HEI System and Ford’s new DuraSpark III system would incorporate computer controlled timing, while Chrysler’s lean burn was upgraded from analog to digital control. All three would work with computer controlled carburetors, knock sensors and oxygen sensors. However, the writing was on the wall, and distributor controlled ignitions quickly became obsolete and eventually would be replaced by distributorless ignition systems that remain in use today.
A special thanks to Daniel Stern for supplying some of the research material on vintage ignition systems